The present invention relates to a color reproduction system having a color image processing unit for estimating colors of a photographed subject from photographic signals obtained by photographing the subject by a color photographing unit such as a digital camera or the like.
As a method of estimating color information on a photographed subject under a predetermined illumination light from color-image data obtained by photographing the subject by a color photographing unit such as a digital camera or the like, there has been proposed a method of using foresight information on the photographed subject such as a base function or a correlation matrix of a spectral reflectance of the subject, as disclosed in Japanese Patent Application KOKAI Publication No. 11-85952. When a base function or a correlation matrix of a spectral reflectance of the subject is used, it is possible to estimate colors of the subject in high precision and reproduce the colors even if a photographic illumination light at the time of the photographing is different from an observation illumination light at the time of an observation.
This conventional color reproduction system uses a spectral sensitivity of a color photographing unit, a photographic illumination light spectrum and an observation illumination light spectrum as information for estimating colors. However, these data have conventionally been obtained by measurement using an expensive measuring instrument such as a spectrometer. Therefore, it has not been possible to provide a simple color reproduction system.
On the other hand, in Jpn. Pat. Appln. KOKAI Publication No. 11-96333, there has been disclosed another method of utilizing a photographic signal of an object such as color charts of known spectral reflectances obtained by photographing the object at the same time as the photographing of a subject or under the same photographing conditions. This method is for estimating color information of the photographed subject under a predetermined illumination light by utilizing the object photographic signal, without using the spectral sensitivity of the color photographing unit or measurement data of the photographic illumination light spectrum. According to this method, it is not necessary to measure in advance the spectral sensitivity of a color-photographing unit. It is not necessary to measure a photographic illumination light spectrum at the time of photographing, either. Therefore, this method can simplify the system.
For example, it is possible to arrange as follows. At a hospital A, a face of a patient is photographed in colors by a digital camera, and at the same time, a predetermined color chart is photographed. The image data of these photographs are transferred to a hospital B. At the hospital B, the face colors of the patient are converted into face colors of the patient under the observation illumination light at the hospital B, and the converted face colors are displayed on the monitor. When this system is employed, an expensive measuring instrument like a spectrometer is not necessary at the hospital A where the picture is taken. Thus, it becomes easy to obtain the data for accurate reproduction of colors.
In the above-described color reproduction system for converting illumination colors by using the foresight information of a subject, it is essential that the color photographing unit has independent spectral sensitivities by at least the number of base functions of spectral reflectances of the subject in order to make an accurate estimate of colors of the subject. It is known that spectral reflectances of body skin colors can be displayed in high precision by a linear sum of three base functions. However, in general, in order to display all the spectral reflectances of an object in the natural world, more base functions are considered to be necessary. Therefore, in order to carry out a simple color reproduction in high precision by using a general three-band digital camera having three bands of red (R), green (G), and blue (B), there has been a problem that a subject to be photographed is limited to an object that has spectral reflectances that can be approximated by three base functions.
On the other hand, there has been known a color reproduction system for carrying out a color reproduction by selecting a suitable set of an output profile and an input profile for one input image, as shown in FIG. 23. In FIG. 23, an input image from an image input unit 10 is input to a device-independent color converting section 11A of a color correcting section 11. The device-independent color converting section 11A converts colors of the input image into a device-independent color image by referring to an input profile 111A, and outputs the device-independent color image to a device value converting section 11B. The device value converting section 11B converts the device-independent color image into a device value that matches the characteristics of an output unit by referring to an output profile 111B, and outputs the device value as an output image to an image output unit 12.
Japanese Patent Publication No. 2678007 discloses a color information correcting unit for correcting a plurality of pieces of read information of a plurality of color components into corresponding recording information for a plurality of record open image colors based on color correction parameters, in the reading of color images.
Further, Japanese Patent Publication No. 2681181 discloses a color data correcting unit that includes an operating unit for correcting colors by carrying out a matrix calculation in color data that has been obtained by digitizing an input signal.
The above-described conventional color reproduction method in the color reproduction system is for carrying out a color reproduction by selecting a suitable set of an output profile and an input/output profile for one input image. According to this method, colors are reproduced based on the same subject characteristic data for all the objects within the image. In other words, colors are reproduced based on object spectrum statistical data. Therefore, the conversion precision is lowered when a plurality of objects are included in the image.
According to the above-described Japanese Patent Publication No. 2678007 and Japanese Patent Publication No. 2681181, although a matrix is selected based on the colors of RGB values, only colors are corrected, and the colors are not converted into colors under the illumination light of the observation side. Further, as subject characteristic data is not used, colors cannot be reproduced in high precision.